Interesting post overall, but no, BTS isn't a return to anything. The Classic Forsaken and the BTS Forsaken are inherently different, especially since much of what both you and @
Thage say about the Classic Forsaken can be wholly dismissed by just playing the intro, let alone doing quests. A minority is not the majority, outliers are outliers because they do not represent the status quo. They're just the ones that act differently based on the premises provided. The Forsaken intro told you flat out that the Forsaken had no bonds to their allies and were willing to get rid of anyone who got in their way, the site called them harbingers of doom, it takes about three quests before the rogue trainer tells you about the new world order and it's by level 5 that you're testing the new plague on a captive, discounting all the other things you do.
The funniest argument thus far being the one guy who I legit don't recall the username of claiming that it's precisely Calia's lack of any link to the Forsaken that makes her fit to be in them because after all they're different and the Forsaken are different, right? This and your otherwise interesting post hit on the same point that @
Aucald runs into hard, namely the idea about how these races aren't homogenous is being used to push a sentiment that pretty much every outcrop and identity are equally valid and expressive of the race, once again in the same way the Highborne Duo on the forums point out an elf using arcane and decide that constitutes a highly essential element of the present night elf identity. The baseline and the status quo are essential - they constitute the core of the race. In the same way that a tauren can choose to be violent towards outsiders this doesn't mean that tauren in general aren't gentle giants and that the Forsaken value free will doesn't mean that every attitude is equally supported or relevant to their identity and that a handful characters suddenly hold just as much weight as the majority.
This is especially the case when, as with your post and as with Aucald, the fact that the Forsaken are shown to be people rather than comically evil 24/7 is used as some kind of gotcha about how actually the BTS Forsaken are entirely on theme because they too are nice, disregarding context and purpose. Someone like Darkmar bailing on the Forsaken in a society that values free will and self-expression are defied and so he acts of his own accord with like minded people, one person honoring their ally's last wish (cheers for reminding me of that quest by the way) while another is scornful are leagues apart from a society of victims operating within a police state unable and unwilling to alter their circumstances. Darkmar left knowing the mages would take issue with him and he did so of his own free will. He did not sit on his ass until the humans that actually were willing to love him all along bailed him out. The Forsaken did use the Light before, yes, but they did so knowing it burnt them and with this being a small group (Existing almost entirely in a CDEv comment in fact), whereas the actual main Forsaken religious institution was the Forgotten Shadow, better tailored towards their self-determination and in the RPG especially, tailored around their undeath and the transhumanist aspect that later shows up in a different way in Cataclysm. They did not do so because the Light actually had no adverse effects like it does with the nu-undead and Faol. To show humanity, solidarity and kindness within undeath is only in any way a relevant trait if undeath itself is a relevant trait, as OP points out. If undeath is not relevant, as it isn't for BTS, if they lack initiative, as they do post-BTS, and if the Light, humanity and such like aren't actually in any way separate from them, then this vastly changes the events themselves because they exist in a wholly separate context.
The Forsaken already had an identity separate from Sylvanas, but they also had one complementary to her. Whereas the previous Forsaken could oppose Sylvanas or support Sylvanas based on the factualities of their race and as already stated opposed their leader far more than any prior race, they also aligned with her both for historical reasons and because her experience and their experience were very close and she was in keeping with the baseline identity. By contrast, the nu-undead never militarily oppose Sylvanas and are shown incapable of doing this on their own, reliant on others to make decisions for them despite their relationship with her and the relationship of the pre-BTS Forsaken with her being completely different - one is transactional but positive, the other is entirely abusive and has been for fifteen years, as well as based on false pretext. The will of the Forsaken has no meaning if there is no adversity and if it isn't actually expressed. Sylvanas, a caricature by this point past any repair, has her last comment about the Forsaken be that they're pitiable sadsacks and while the narrative feigns otherwise, she's completely correct. No sooner has she departed, having faced zero Forsaken opposition from anyone except Zelling who's been Forsaken for five minutes and was spurred by Baine but does Voss say that most of them are in ennui and can do nothing to resolve their problems, and neither can Voss help them, despite all her development - she needs an outside party in Calia to fix them. This despite there being no adversity to them, no rejection by anyone, human or cosmic, no limiting effects of undeath, nothing. The titular will of the Forsaken is entirely absent. The antagonist's minions get to actually deal with every aspect thus far mentioned in the new book, relevant as they are to the undead experience, far outstripping the Horde-aligned undead in screen time not just when it comes to their plot function, but in terms of the information on their condition, past and thought process. As to the sloppy seconds, they get to be occupied by Calia off-screen with nary a comment. And why would there be a comment? They've nothing to say because there's nothing there. They're flotsam that has to be there for gameplay reasons. The appendix of races - evolved to the point of uselessness, but most of the time not worth to even get rid of.